THE STRUCTURE OF THE THYROGLOBULIN MOLECULE: Our previous electron microscopical studies have demonstrated that thyroglobulin molecules may have either an ovoid or a cylindrical conformation. Ovoid molecules are characteristic of well-iodinated thyroglobulin whereas cylindrical molecules are characteristic of poorly iodinated thyroglobulin. Cylindrical molecules are transformed into ovoid molecules when incubated with peroxidase and an H2O2-generating system. The conditions for this transformation will be further studied in order to answer the questions whether the transformation from cylindrical to oviod conformation is specifically catalyzed by peroxidase and whether this transformation is reversible. Our studies on the significance of the molecular conformation for in vitro iodination and hormone formation will be pursued. RELATION BETWEEN EXOCYTOSIS AND ENDOCYTOSIS AND BETWEEN EXOCYTOSIS AND IODINATION IN THE THYROID: Our previous studies indicate that a prerequisite for endocytosis in the follicle cells is addition of membrane to the apical cell surface by exocytosis. Also, the rapid stimulatory action of TSH on iodinatin seems to depend on stimulated exocytosis. The relation between these processes will be further explored by studying the acute effect of TSH on endocytosis and iodination in follicle cells with a reduced number of exocytotic vesicles and a disturbed intracellular transport of exocytotic vesicles.